Gear oils and industrial oils are required to meet certain exacting performance specifications. They must exhibit long term stability, implying good resistance to oxidation over a wide range of temperatures coupled with other performance properties including good anti-wear performance. Depending upon the specific application, other performance characteristics may be required. For example, in high temperature circulating oils, high temperature stability must be the main requirement while minimum anti-rust performance is necessary since little water is present at high temperatures. However, in other applications, anti-rust performance becomes important, for example, in wet applications such as use in paper-making machinery.
The properties of oils may be differentiated on the basis of whether they are bulk properties which are not affected significantly by contact with the surface of other materials, for example, the components of a machine or surface-related properties which affect and are affected by the surfaces with which the oil is in contact Oxidation resistance, for instance, belongs largely in the fromer category although the rate at which an oil undergoes oxidation in use is affected by the character of the metal surfaces in contact with the oil. Extreme pressure resistance may also be included in this category. Other properties such as anti-corrosion, anti-rust, anti-wear are directly dependent on the nature of the surfaces--usually metal--with which the oil is in contact during use. The properties which are surface dependent impart another consideration into the formulation of a finished lubricant since the additives which are used to improve the properties of the lubricant base stock and provide the desired balance of properties may be in competition for available sites on the metal surface. For this reason, it is often difficult to obtain a good balance between the performance properties which are surface dependent. One instance of this is with anti-wear and anti-rust properties: it is difficult to produce an oil which possesses both properties in good measure at the same time.
Different types of base stocks have different performance characteristics. Ester base stocks, for example, the neopentyl polyol esters such as the pentaerythritol esters of monobasic carboxylic acids, have excellent high performance properties as indicated by their common use in gas turbine lubricants. They also provide excellent anti-wear characteristics when conventional anti-wear additives are present and they do not have any adverse effect on the performance of rust inhibitors. On the other hand, esters have relatively poor hydrolytic stability, undergoing hydrolysis readily in the presence of water at even moderate temperatures. They are, therefore, less well suited for use in wet applications such as paper-making machinery.
Hydrolytic stability can be improved by the use of hydrocarbon base stocks. The use of alkyl aromatics in combination with the other hydrocarbon base stocks such as hydrogenated polyalphaolefin (PAO) synthetic hydrocarbons and the improved hydrolytic stability of these combinations is described, for example, in U.S. Pat. No. 5,602,086, corresponding to EP 496 486. Traditional formulations containing PAO's, however, present other performance problems. Although the hydrolytic stability of hydrocarbon base stocks including PAOs is superior to that of the esters, it is frequently difficult to obtain a good balance of the surface-related properties such as anti-wear and anti-rust because, as noted above, these surface-related properties are dependent upon the extent to which the additives present in the base stock compete for sites on the metal surfaces which they are intended to protect and high quality hydrocarbon base stocks such as PAOs do not favorably interact with the additives used for this purpose. It is therefore continuing problem to produce a good combination of surface-related properties including anti-wear performance and anti-rust performance in synthetic oils based on hydrocarbon base stocks such as PAO's.